European Food Research and Technology

, Volume 234, Issue 4, pp 563–569

Oat fibre: overview on their main biological properties

Authors

    • College of Food Science and TechnologyHebei Normal University of Science and Technology
Review Paper

DOI: 10.1007/s00217-012-1666-2

Cite this article as:
Xu, R. Eur Food Res Technol (2012) 234: 563. doi:10.1007/s00217-012-1666-2

Abstract

Oat is a species of cereal grain grown for its seed and widely accepted to contain many valuable constituents. These include especially fibre that posses important biological properties, particularly with regard to promotion of health, as well as to prevention of diseases. Reducing risk of coronary heart disease and plasma LDL cholesterol has, indeed, been associated with such oat fibre. The most important advances reported to date pertaining to biological properties of oat fibre are reviewed in this communication. The purpose of this review is to focus on available knowledge on the interactions between insoluble or soluble dietary fibre and lipid metabolism in the human body.

Keywords

Dietary fibreOatFunctional foodHealth benefits

Introduction

Oats are generally considered “healthy”, or a health food, being touted commercially as nutritious. The discovery of the healthy and cholesterol-lowering properties has led to wider appreciation of oats as human food. Oats contain more soluble fibre than any other grain, resulting in slower digestion and an extended sensation of fullness. The popularity of oatmeal and other oat products again increased after 1998 decision by the FDA, noting that 3 g of soluble fibre daily from whole oats may reduce the risk of heart disease. The soluble fibre in whole oats comprises a class of polysaccharides known as beta-d-glucans. One type of soluble fibre, beta-glucans, has been proved to help lower cholesterol. Beta-d-glucans, usually referred to as beta-glucans, comprise a class of indigestible polysaccharides widely found in nature sources such as grains, barely and mushrooms. In oats, barley and other cereal grains, they are located primarily in the endosperm cell wall.

Oat beta-glucan is a soluble fibre. It is a viscous polysaccharide made up of units of the monosaccharide d-glucose. Oat beta-glucan is composed of mixed-linkage polysaccharides. This means the bonds between the d-glucose or d-glucopyranosyl units are either beta-1, 3 linkages or beta-1, 4 linkages. The 1, 3-linkages break up the uniform structure of the beta-d-glucan molecule and make it soluble and flexible. In comparison, the indigestible polysaccharide fibre is also a beta-glucan, but is not soluble. The reason it is insoluble is that fibre consists only of 1, 4-beta-d-linkages.

Both epidemiologic and clinical studies suggest that dietary factors in addition to the intake of fat and cholesterol influence the degree of risk of coronary heart disease (CHD) [73]. Human experiments have clearly shown that oat fibre tends to lower plasma total and LDL cholesterol. Cereal foods with low glycaemic index such as oats are beneficial for people with diabetes and might lower plasma lipids. I will discuss the effects of oat fibre on human health, describing and evaluating their significance and the possible mechanisms of action.

Plasma cholesterol levels and Lipid digestion

Studies with animals and human subjects indicate that there is a considerable inter-individual variation in the rate of dietary cholesterol absorption in the small intestine [61]. Furthermore, intestinal cholesterol absorption efficiency is positively correlated with plasma cholesterol levels [39], and plasma cholesterol level is positively correlated with the incidence of CHD [13]. It shows that a reduction in cholesterol absorption in the small intestine should be beneficial in the prevention of CHD. Dietary fibres are nondigestible, water-soluble or water-insoluble carbohydrates and are claimed to have beneficial effects on the lowering of serum cholesterol and the prevention of CHD [19]. The impressive effects of the fibre investigated in the study, for example, the satiation and satiety effects of cellulose, were sufficient to stabilize serum cholesterol levels against the increase induced by the high-fat/high-cholesterol (HFHC) diet and reduce the degree to which liver cholesterol stores are increased by the HFHC diet [75]. However, the dose of fibres used was higher than doses usually used in human trials. A possible beneficial effect of fibres on cardiovascular risk might be related to the different mechanisms by which the fibres influence lipid metabolism. Feeding of cellulose partially could prevent accumulation of cholesterol in hepatic stores [75]. One mechanism [22] involves binding of bile acid molecules in the intestinal lumen to the fibre. But the neutral fibre such as cellulose is not for this explanation. The mechanism of action of dietary fibres may relate to their ability to suppress energy intake. Dietary fibres could induce both satiety and satiation [10]. These two effects could reduce the intake of cholesterol present in the diet. Knowledge about the mechanisms by which the different fibres prevent cholesterol accumulation will be helpful for investigating the clinical effects of fibre therapy. Investigation of cellulose in clinical studies is most intriguing, because a hypocholesterolaemic effect of cellulose has not been established as clearly [14], although intake of high concentrations of cellulose as a food supplement in man was reported to lower serum cholesterol by about 25% [62].

Researchers who have tried the effect of eating increased oat fibre have found reductions in plasma total and LDL cholesterol [9, 54]. However, in some studies, there was no significant change in total or LDL cholesterol [47, 68, 72]. Some trials showing no effect were conducted with free-living out patients, who had quite low plasma cholesterols and took higher fat and energy in the oat bran period [69]. This raises the possibility that the oat bran used in those trials was low in oat fibre. Few of the published trials reported the fibre composition of the oat [74]. On the other hand, the monounsaturated and polyunsaturated fatty acids in oats (about 4.7 g/100 g) may contribute to the cholesterol-lowering effect. Ruottinen et al. [58] found that dietary fiber more effectively lowers serum cholesterol concentrations when the saturated fatty acid intake is low.

The effect of dietary fibre on serum cholesterol concentrations may be largely mediated by the enhanced faecal excretion of bile acids [35]. Kretsch et al. [41] reported that the fibre from oat bran could increase the faecal output of bile acids. Mucilaginous β-glucan forms gels in the small intestine that could interfere with the absorption of both cholesterol and bile acids. Other fibres may exert their effects on the faecal output of acidic sterols by adsorbing less polar bile acids in the colon [35]. One research reported that the increased faecal output of bile acids due to fibre is not always accompanied by a reduction in serum cholesterol levels [76]. This suggests that the reduction in serum cholesterol levels by dietary fibre may be due to a number of events that interact to result in an increased faecal steroid excretion not fully compensated for by de novo cholesterol synthesis.

The active part of oats total fibre is the β-glucan. It is the main component of oat gum. Increasing the viscosity of the intestinal content to a sufficient extent alters organ motility, potentially decreases intraluminal mixing and increases the thickness of the unstirred water layer at the intestinal mucosa. These combined effects likely explain the observed reduced rates of the intestinal uptake of cholesterol and fatty acids in the presence of viscous fibres [24]. Concentrated oat gum lowers plasma cholesterol in animals [78] and humans [9]. When oat bran was treated with β-glucanase, the cholesterol-lowering action in rats was abolished [71]. In one study [43], adding 10 g of oat bran into a meal led to a 37, 43 and 31% lowering of postprandial chylomicron triacylglycerol, cholesterol or phospholipid responses, respectively. Therefore, above studies support the concept that some fibre sources can, by altering lipid processing in the gut and probably resecretion, lower the accumulation of intestinally derived chylomicrons in the circulation postprandially.

Oat β-glucan, a water-soluble viscous polysaccharide, has been linked with many health benefits, such as reduced cholesterol levels and balanced blood glucose and insulin levels [82]. Insulin has been reported to increase both cholesterol synthesis [8] and the hepatic synthesis and secretion of very-low-density lipoprotein [56]. Hlebowicz et al. [29] reported that the postprandial delta blood glucose level was statistically significantly lower at 40 and 120 min after the cereal bran flakes meal. However, lower postprandial glucose and insulin concentrations have not been shown to be caused by the fermentation of β-glucan in the colon [6].

A number of in vitro studies have shown that dietary fibres can alter the lipolysis process. One mechanism has been shown. The soluble fibres forming viscous solutions drastically reduce the rate of lipid emulsification, with a resulting noticeable lowering of the extent of fat lipolysis [52]. This has been confirmed with oat bran [43]. Other fibres, such as chitosan, can generate aggregates with lipid globules and consequently lower the extent of lipolysis [3]. It has been recognized for a long time that a diet enriched in oat bran fibres can significantly increase faecal fat excretion in humans [2, 43].

Diabetes

To prevent the development of diabetes mellitus, the American Diabetes Association recommends a reduction in caloric intake and increased consumption of dietary fibre (14 g fibre/1 kcal) and food containing whole grain [5]. An increased intake of fibre has been shown to reduce the risk of diabetes [50, 60]. A diet high in fiber lowers serum insulin concentrations and reduces the risk of obesity [63], thus reducing the risk of type 2 diabetes. High dietary fiber intake also protects against some types of cancer [59]. Fibre has been shown to reduce the glycaemic response in healthy subjects [7]. The intake of total dietary fibre, particularly insoluble and cereal fibres, has been shown to have an inverse association with diabetes type 2 [50]. It has been assumed that fibre fermented in the colon by the bacterial flora releases short-chain fatty acids, thus lowering the postprandial glucose levels [55, 77].

The effect of β-glucan is not fully understood. Products enriched with β-glucan have been shown to reduce postprandial glucose and insulinaemic responses in healthy men [12, 48] and in type 2 diabetes patients [31, 70]. Reduced postprandial glucose and insulin concentrations after consumption of types of fibre have been shown to be caused by delayed mouth-caecum transit and delayed absorption of glucose in the small intestine [32]. Oat gum, an oat extract composed of β-glucan, has been shown to cause a reduction in plasma glucose and insulin levels [81].

Satiety

The amount of dietary fibre in the diet has been linked to the reduction in energy intake and body weight as well as increased satiety [64]. Oat β-glucan has been suggested to be crucial for their effect on satiety-related attributes [64]. Burton-Freeman [10] and Howarth et al. [30] discuss several possible mechanisms behind the influence of soluble viscous fibres on food intake regulation: the slower gastric emptying time because of increased viscosity of the chyme in the gut as well as longer small intestine passage time and the absorption rate of nutrients, both of which are believed to increase satiety by enhancing satiety-mediating signals to the central nervous system [46]. Howarth et al. [30] suggested direct effect of dietary fibre on energy intake is the decreased absorption of macronutrients because of the physical barrier formed by dietary fibre. One study has shown that the viscosity of soluble fibres and oat β-glucan had a role in their ability to increase satiety [45]. Fibre consumption has been shown to be associated with a reduced risk of overweight in children [28], but this assumption was not supported in the study by Ruottinen et al. [58]. There are several explanations for this phenomenon. Dietary fiber intake decreases overall food intake by promoting satiation via different mechanisms [63].

Solid foods have been found to increase satiety and decrease hunger more effectively than liquid foods [40]; it is possible that the larger satiating effect of solid food in itself drowns the weaker satiating potential of fibre. In beverages, however, fibre could be used to boost their satiating capacity closer to the level of solid or semisolid foods, or even above it, which was the case in the study by Lyly and their team [46], when compared to white wheat bread. This could offer an interesting possibility for the beverage industry to produce functional beverages with improved effect of liquid calories on satiety [46]. Naumann et al. [51] showed that when incorporated into a fruit drink, β-glucan lowers serum concentrations of total and LDL cholesterol. A reduced cholesterol absorption contributes to the cholesterol-lowering effect of β-glucan without affecting plasma concentrations of lipid-soluble antioxidants.

Colonic function

Oat hull fibre is an insoluble source of dietary fibre, derived from the outermost layer of the oat grain, made up mainly of cellulose. Traditionally, oat hulls have been discarded during processing, but the need for concentrated, insoluble fibre sources for human consumption has resulted in the production of oat hull fibre for human food use. Insoluble fibre is necessary in the human diet to maintain healthy colonic function and reduce constipation. One research reported that oat full fibre is resistant to fermentation in the human colon, has no effects on serum lipids [67]. The analysis of faecal samples in the above study indicated that oat hull fibre travels through the entire gastrointestinal tract undigested and nonfermented. The speed of transit through the intestinal tract affects nonstarch polysaccharide fermentability, particularly when transit is fast [67]. This may explain why the oat hull fibre was totally undegraded. It may be possible that some fermentation of oat hull fibre might occur in individuals who have very slow transit times [67].

Clinical significance of dietary fibre intake

Mucilaginous fibres such as guar and oat bran have been used in the treatment for hypercholesterolaemia [1, 33]. However, these fibres have not been shown to have therapeutic advantage over cholestyramine or other conventional hypocholesterolaemic agents.

Some types of dietary fibre exert direct effects on the total serum cholesterol concentration. There is general agreement that possible changes in serum lipid levels due to the alterations in fibre intake are quantitatively minor in comparison with the effects of changes in the dietary intake of fatty acids and cholesterol [35]. Mattson et al. [49] have suggested that a reduction in the total fat intake with an increase in the polyunsaturated fraction and a decrease in the dietary cholesterol intake could be expected to reduce the total serum cholesterol concentration. More information is needed on the effects of various types of dietary fibres on the levels of serum lipoproteins. Reports of epidemiologic studies in Western countries have suggested that dietary fibre intake offers protection against CHD [42, 53]. The inverse associations with mortality from CHD were observed for both IDF (insoluble dietary fibre) and SDF (soluble dietary fibre) intakes [18]. The inverse association between dietary fiber and CHD can be explained by different mechanisms, such as lowering blood pressure [37] via reduction in abdominal obesity and improvement of vascular reactivity [34], all of which may prevent or delay the development of atherosclerosis. Previous studies suggested that those effects were derived by both IDF and SDF [42, 53]. However, SDF may have a stronger cholesterol-lowering effect [11], whereas IDF may have a stronger clotting factor reduction effect [44]. The apparent protective effect of fiber on risk of CHD may be due to other health-related habits, such as regular exercise, no smoking.

β-Glucan is composed of branched chains. Functional foods enriched with β-glucan from mainly oats are widely available to decrease serum LDL cholesterol. Some studies found reductions greater than 10% [25], but a few studies demonstrated virtually no benefit [27]. Inconsistencies in the reported effects of oat products may be due to several factors, such as mode of administration, or differences in solubility or molecular weight. Kerckhoffs et al. [38] suggested that the food matrix or the food processing affects the hypocholesterolaemic properties of β-glucan. And this potential benefit may be limited by the amount of fiber that individuals would need to consume. So the physicochemical properties of oat β-glucan should be considered when assessing the cholesterol-lowering ability of oat-containing products. Water-solubility and molecular weight of β-glucan may also influence its hypocholesterolaemic effect. There are some indications that the molecular weight of β-glucan may partly be reduced during its passage through the upper gastrointestinal tract [26]. However, controversial reports are available that show no quantitative losses of β-glucan from oat flour or bran in the stomach and in upper parts of the small intestine [4]. During passage through the stomach and small intestine, insoluble β-glucans are partly converted into soluble form and can form viscous solutions in the gut. The viscosity of β-glucan solutions is determined by its molecular weight and solubility [80], both of which can be altered by normal methods of processing and storage [57]. A higher intestinal viscosity lowers the reabsorption of bile acids, leading to an increased excretion of bile acids. Increased bile acid excretion promotes bile acid synthesis from cholesterol, which will increase LDL cholesterol uptake in the liver. Wolever et al. [79] showed consumption of 3 g of high-molecular-weight (MW) oat β-glucan daily in a ready-to-eat cereal reduced LDL cholesterol by 0.21 mmol/L. Also, the effect of oat β-glucan on LDL cholesterol was significantly related to its viscosity, which was determined by MW and bioavailability. Meta-analyses of the statin trials [15] suggest that lowering LDL cholesterol by 0.2 mmol/L would reduce the risk of CAD (coronary artery disease) by about 5%. Some studies compared the cholesterol-lowering effects of cereal β-glucans at different doses and molecular weights [36, 66], but they provide contradictory or equivocal results. Possible reasons are that the β-glucan was not completely solubilized in the intestine or that the molecular weight of the β-glucan was reduced by processing. A high molecular weight may be associated with an increased intestinal viscosity, which may decrease the absorption of cholesterol. However, it was shown that in healthy volunteers, both high- and low-molecular-weight β-glucan reduced serum concentrations of total and LDL cholesterol equally in relation to baseline [21]. These results suggest that molecular weight alone cannot predict the cholesterol-lowering effects of β-glucan.

As the intake of dietary fibres is increasing in daily life, it is necessary to know how these fibres interact with administered drugs. Fibre administration can reduce the severity of dopamine-mediated gastrointestinal and cardiovascular adverse effects by providing lower levodopa maximum concentrations and also provide a more uniform response by maintaining plasma concentrations in a narrower range, when levodopa was administered alone [23] and in the presence of carbidopa [20]. A further study showed that there was an improvement in the extent of levodopa absorbed with higher final concentrations using repeated doses [16]. A recent study concluded that the administration of the drug improves the benefits of dietary fibre by preventing its bacterial degradation [17].

Conclusion

Nutrition research on human cereal consumption is to increase understanding of the mechanisms for the protectiveness of whole grains [65]. Thus, in future studies, the effect on cardiovascular morbidity and mortality as well as the occurrence of side effects should be evaluated separately for each kind of fibre. Our scientific knowledge in this area is still limited, and more extensive research, as well as further metabolic consequences, is required to better understand the mechanisms occurring during meal processing.

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© Springer-Verlag 2012